1. Field of the Invention
This invention relates to data reproducing apparatus and, more particularly, to such apparatus which can be used to play back data having different recording densities, thereby reducing the bit error rate which otherwise may accompany data that had been recorded with a higher (or lower) density.
2. Description of the Prior Art
In one type of data recording/reproducing system, audio information is digitized, as by PCM encoding, and recorded by one or more (preferably, at least two) rotary transducers which helically scan oblique tracks across a moving magnetic tape. Such systems may record and play back only audio signals, such as the so-called R-DAT systems, and others record and play back both audio and video signals. One type of video tape recorder (VTR) in which audio information is PCM encoded is the so-called 8-mm format. In an 8-mm VTR, two (or more) rotary heads scan adjacent tracks across the video tape; and each track includes a video information portion and a separate audio information portion. An example of such an 8-mm format is illustrated in FIG. 11 of the accompanying drawings.
As shown in FIG. 11, alternate tracks TRA and TRB are scanned, and video information is recorded in the video portion TR.sub.vd of each track and PCM-encoded audio information is recorded in the audio portion TR.sub.ad of the same tracks. Preferably, the PCM-encoded audio signal is time base compressed. The resultant PCM-encoded, compressed signal is recorded by what is known as biphase-mark modulation; and this biphase-mark modulated PCM, compressed audio signal is recorded on the audio portion TR.sub.ad of each track TRA, TRB, as described in U.S. Pat. No. 4,551,771, assigned to the assignee of the present invention.
In biphase-mark modulation, the binary signals "1" and "0" are represented by different frequencies. For example, and as shown in FIG. 12A of the accompanying drawings, a signal S.sub.1 having a relatively lower frequency f.sub.1 (on the order of about 2.9 MHz) and a signal S.sub.2 having a relatively higher frequency f.sub.2 (on the order of about 5.8 MHz) are recorded to represent a binary "0" and a binary "1", respectively. The relationship between the frequencies f.sub.1 and f.sub.2 and phases of the signals S.sub.1 and S.sub.2 is such that the zero crossing points of signal S.sub.1 coincide with the zero crossing points of the signal S.sub.2. These zero crossing points are used during a playback operation to generate sampling pulses which, in turn, are used to discriminate between signals S.sub.1 and S.sub.2 and thus demodulate the played back signals into digital "0" and "1" bits.
One technique to generate the aforementioned sampling pulses is to synchronize a phase locked loop with the played back signals S.sub.1 and S.sub.2. As an example, the phase locked loop may generate clock pulses at a frequency four times the frequency f.sub.2 (and, thus, eight times the frequency f.sub.1), with the phase locked loop being phase synchronized with the zero crossings of the signals S.sub.2 and S.sub.1, depending upon which is played back. FIG. 12B of the accompanying drawings illustrates the clock signals generated by the phase locked loop in response to signals S.sub.1 and S.sub.2. Then, selected ones of these clock signals are extracted, as by a gating circuit, to obtain the sampling pulses P.sub.s shown in FIG. 12C of the accompanying drawings. By comparing the waveforms shown in FIG. 12A with the sampling pulses illustrated in FIG. 12C, it is seen that these sampling pulses occur at 45.degree., 135.degree., 225.degree. and 315.degree., relative to signal S.sub.1 ; and these same sampling pulses occur at 90.degree. and 270.degree. with respect to signal S.sub.2.
When the signal S.sub.1 is sampled by the sampling pulses P.sub.s, it is recognized that a positive level is sampled by two successive sampling pulses and then a negative level is sampled by the next two sampling pulses, followed by a positive level sampled by the next-following two sampling pulses, and so on. However, when the signal S.sub.2 is sampled by these very same sampling pulses P.sub.s, positive and negative levels are produced in response to successive sampling pulses. Thus, by reason of this sampling, signal S.sub.1 can be distinguished from signal S.sub.2 and, consequently, "0"s and "1"s can be discriminated. Hence, this sampling technique serves to demodulate a biphase-mark modulated signal. Further description of the reproduction of biphase-mark modulated data is found in copending application Ser. No. 105,830, corresponding to European Published Application No. 264228, both assigned to the assignee of the present invention.
The video information normally recorded on a VTR, and particularly a VTR of the 8-mm format, includes a luminance component that is frequency modulated to a relatively higher frequency band. If biphase-mark modulation is not used, an FM audio signal also is recorded, this audio signal having a frequency band that is less than the FM luminance signal band. In addition, a chrominance signal shifted to a frequency band that is even lower than that of the FM audio signal is recorded, as well as an automatic track follower signal. The frequency spectrum of these signals is illustrated in FIG. 13 of the accompanying drawings, in which a recorded composite television signal S.sub.vd is comprised of the FM luminance signal S1V, the FM audio signal S2V, the chrominance component S3V and the automatic track follower signal S.sub.ATF. All of these signals are superposed and recorded in the alternate tracks shown in FIG. 11.
When recording a PCM audio signal, the same magnetic head that is used to record the composite television signals also records the PCM signal. The frequency spectrum of this PCM signal also is illustrated in FIG. 13 as the PCM audio signal S.sub.ADNR. Here, the PCM audio signal S.sub.ADNR is recorded with biphase mark modulation; and it is seen from FIG. 13 that this signal S.sub.ADNR has a peak value at a frequency which substantially coincides with (or at least approximates) the center frequency of the FM luminance signal band S1V. Since the peak value frequency of the PCM audio signal S.sub.ADNR is close to the center frequency of the FM luminance band S1V, and since the spectrum of the PCM audio signal S.sub.ADNR is substantially within the overall spectrum of the composite television signal S.sub.vd (except for higher frequency components having relatively low signal levels), the PCM audio signal is subjected to approximately the same degree of azimuth loss as is exhibited by the video signal S.sub.vd. As is known, azimuth loss is the phenomenon by which a signal that is recorded by a head having one azimuth angle is substantially attenuated when played back by a head having a different azimuth angle. Azimuth loss is more pronounced at higher frequencies than at lower frequencies. Nevertheless, because the PCM audio signal S.sub.ADNR is subjected to azimuth loss, adjacent tracks of both video and audio signals, such as shown in FIG. 11, can be recorded without separating guard bands therebetween. Even without guard bands, azimuth loss minimizes cross-talk in both the video and audio signals which are picked up from an adjacent track. By avoiding guard bands, desirably higher recording densities are attained.
Although the same basic format may be used to record data, and particularly PCM audio information, recording densities can be further improved if higher sampling, or clock, frequencies are used to encode, modulate and record the data. However, it is desired that a particular recording/reproducing device be compatible with various different types of encoding and modulation techniques which improve the recording densities. For example, it is preferred that a given 8-mm VTR be capable of playing back PCM audio data that may be recorded with so-called normal densities or with higher recording densities. For example, the very same data reproducing device should be compatible with PCM-encoded audio information that is recorded by biphase-mark modulation or that is recorded with higher recording densities, for example, by so-called 8-10 modulation. Examples of 8-10 modulation and the advantages attained thereby are described in U.S. Pat. Nos. 4,577,180 and 4,617,552, both assigned to the assignee of the present invention. It is appreciated that, by increasing the recording density of the PCM audio signal, as may be attained by 8-10 encoding, a greater amount of data may be recorded, thereby improving the resolution and, thus, the quality of the audio signal which, ultimately, is played back with higher fidelity.